In the application of large, line start, induction motors it is necessary to know the rotor temperature at shutdown in order to determine the time at which the motor may be restarted.
Typically, to determine this temperature, thermocouples embedded in the rotor next to a rotor bar are coupled to a battery operated telemetry system on the motor shaft. If the motor interior is accessible, contact probes are sometimes inserted or an infra-red scanner (with great uncertainty) can be used. It is usually impractical or difficult to get at the motor interior to measure rotor temperature. When knowledge of rotor temperature is essential and the motor is large enough, embedded sensors and telemetry are often used. It would thus be desirable to provide a low-cost, non-invasive method of estimating rotor temperature sufficiently well to obviate need for thermocouple telemetry or other invasive probes. Such method would desirably be available through a means that is convenient and easily enough obtainable to warrant installation on a broad range of motors extended to smaller horsepower ratings.